1. Field of the Invention
The present invention relates to controls for controlling the speed of AC electrical motors.
2. Description of the Prior Art
One common type of speed control for AC electric motors involves the use of a gated bilateral switch such as a triac which is connected in series with the motor. The triac may be triggered or gated at different phase angles of each half-cycle of the input AC line voltage in order to vary the voltage applied to the motor and therefore the motor speed.
In many applications, it is desirable to be able to control the speed of an electric motor over a wide range of speeds. One such application is in fan control systems for controlling temperature in a commercial confined stock growing operation for raising livestock such as hogs. In typical operations, the animal waste drops through the floor and into a pit below the floor. Constant ventilation must be maintained at a low speed so that the methane gas generated by the animal waste is dissipated, and does not endanger the livestock. As temperature increases in the building, fan speed should increase to increase ventilation and help reduce the temperature. In this type of system, therefore, the electric motor driving the fan must operate at a constant, very low speed under most conditions, but must be capable of much higher speeds depending upon the temperature.
One problem which has been encountered with prior art system phase angle firing motor controls is that there has been a tendency of the electric motors (particularly permanent split capacitor or shaded pole single-phase motors) to stall at low speed as the temperature increases. This is clearly undesirable, since it limits severely the range of voltages and speeds over which the motor may operate. In systems of the type described above, operation over a wide range of speeds without stalling is a necessity.
Another problem with the prior art motor control circuits has been encountered when the temperature sensor was remotely located with respect to the remainder of the circuit. There has been an increasing tendency of the motor to stall, and this tendency appears to be affected by the length of the leads running from the temperature sensor to the remainder of the control circuit. In many applications, it is highly desirable to position the temperature sensor in a remote location with respect to the motor control circuit and the motor. Limitations on sensor lead length severely limit the usefulness of the motor control circuit.